Except for when I travel, I don't much use gmail and Google search. (When traveling, I like booking using gmail, auto calendar and travel events, notifications, etc.). Otherwise I use my own email, calendar, etc. and DuckDuckGo.
I do pay Google about $25/month directly for Google Play Music, renting movies, and extra GDrive storage.
If I were a typical customer, would a $25/month average spend per user be enough to pay their expenses and generate sufficient profit?
Edit: dividing $4 billion per month by $25 per user per month yields a required 160 million paying users per month to equal ad revenues.